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Live Help. A helical spring washer fails to prevent self-loosening. A video of a typical test is shown below. Helical Spring Lock Washers Helical spring lock washers have been in use for well over years.
Any more force and the bolt will enter the plastic region and permanently deform. Since any additional force will begin to yield the bolt, you want to give yourself some margin for error. Engineers select a bolt tension that is somewhere between the calculable minimum functional clamping force and the yielding force…. It is actually quite difficult to determine the exact load the fastener sees during clamping.
We now know how important it is to avoid over tightening a bolt, but how do we know when it is yielding? For everyday purposes the clamping force can be approximated by measuring the tightening torque.
You can look up the recommended tightening torque for a given fastener size in my bolt sizing calculator or in a table like the one found here. Those methods work OK for most things, but some critical applications require you to be certain of the clamping force think spacecraft or large weights above your head.
The torque method has difficultly accounting for friction and lubrication, but at least the torque is mathematically correlated to the clamping force. There are better options though. Load indicating washers can accurately verify bolting loads by squishing open a paint sack after reaching a specific load. The drawback with these is that they only work once. This is by far the most accurate method of measuring bolt clamping load.
On the other hand, a single box of these bolts can cost around 10 times as much as a standard fastener! Bolts actually stretch very slightly when force is applied, which causes the loading on each thread to be different. Because of this stretch, when you apply a tensile load on a threaded fastener the first thread at the point of connection sees the highest percentage of the load.
The load on each successive thread decreases from there, as seen in the table below. Additional threads beyond the sixth will not further distribute the load and will not make the connection any stronger.
So will a bolt break before the nut strips? Nuts typically have no less than three internal threads, but nut thickness standards have been selected on the basis that the bolt will always sustain tensile fracture before the nut will strip. Have you ever seen a fastener labeled with a 2A or 3B rating and wondered what that meant? That number-letter combo is used to indicate the thread class of the fastener. Thread classes include loose to tight , A external , and B internal. These ratings are clearance fits which indicates the level of interference during assembly.
All fasteners are available with either coarse or fine threads and each option has its own distinct advantages. Finely threaded bolts have slightly larger cross-sectional areas than coarse bolts of the same diameter, so if you are limited on the bolt size due to dimensional constraints, choose a fine thread for greater strength. Fine threads are also a better choice when threading a thin walled member.
Fine threads also permit greater adjustment accuracy by requiring more rotations to move linearly. On the other hand, coarsely threaded bolts are less likely to be cross threaded during assembly. They also allow for quicker assembly and disassembly, so choose these when you will be reassembling a part often.
Coarse ly threaded fasteners are much more commonly available in the united states. Would you expect a bolt to be stronger or weaker at very high temperatures?
How about at cryogenic temperatures? Why would steel be strongest at whatever typical room temperature happens to be? Room temperature is just another non-extreme point on the curve. You can make bolted connections more resistant to shear loads by using clever design instead of larger bolts. For maximum strength, try to use the correct thread length for the connection. In the image below you can see two connections which are identical except that the one on the right has a properly sized thread length.
It exposes the bolt shank rather than the threads to the applied load at the connection seam. All else remaining the same, the connection on the right will be stronger because the shank has a larger cross-sectional area and no stress concentrations. Another clever trick is to design connections so that the applied load will be on multiple sections of the bolt, as opposed to just one section. In the images below there are two connections. The one on the right is twice as strong as the one on the left because it would have to shear the bolt off in two places to become free.
Also, the single shear configuration can also lead to bending loads on the fastener and loosening of the connection see 1. Have you ever cursed the day you were born because you just stripped out a Phillips head screw?
As frustrating as that is, it turns out that Phillips head screws are designed to strip out via the tapered point and rounded edges. The technical term is called a cam-out and every time it happens the relative surface motion wears out your screw. Alternate screw heads like torx and pozidriv are specifically designed not to cam-out. Actually every book by Carroll Smith is pure gold.
I think the spring washer actually at least in current usage is to increase preload with a controlled deformation. When you have say a safety item with This causes a preload that puts the energy back into the tightening torque and not the strain energy of the steel. Basically it controls your torque limit so you can keep it fastened without torquing the nut but still maintaining the preload to prevent loosening from NVH or light brushing torques.
Basically, lower loosening and install torques but still same preloads and tolerancing. Like Like. Like Liked by 1 person. Its an interesting piece of trivia for sure, and Im always looking to pick up more if you know of any. Spring washer is very good for the correct purpose: when the fastener is tightened as necessary only, when actually no need such fully tightened.
Spring washer will keep the bolt in place. I believe the nut you are discussing are called castellated nuts. Then a pin goes through to lock them in place. Torque wrench for lug nuts is a must though.
There is a whole class of nuts called deformed or distorted thread lock nut. By punching the edge or face of a nut you can deform the threads for an interference fit with the bolt. The punching leaves those telltale squares or triangles on the nut. Much more subtle than a castellated nut, which rely on a cotter pin for its locking action. One of those things is about fastener.
The subject of nuts and bolts may be a simple one for mechanical engineers, however. As a subject […]. First, thank you for the article. Second, I found on youtube this video also from boltscience. From what I see the test shows now that helical spring washers do help. The amplitude in change but still.
Can you please check out the video, what do you make of this? Some fixings are lubricated thus changing to coefficient of friction and some are not, so you cannot compare each test. Secondly using stainless steel for testing purposes is wrong, as you are trying to get to your pre-load you have galling cold welding. This other test conducted by Nord — Lock shows again that spring washers do have an positive effect. What about the general use of flat washers?
Washers are used to provide a bearing surface for a nut or screw head, cover large clearance holes, and distribute fastener loads over a large area, particularly on soft materials. They are useful hardware for many reasons but are not intended as a reliable source for vibration protection.
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